A Doherty amplifier configuration is known as a configuration to achieve high efficiency of a power amplifier, in which two amplification circuits of different operation classes are arranged in parallel (e.g., see Non-Patent literature 1). FIG. 10 shows a basic configuration of a conventional Doherty amplifier.
As shown in FIG. 10, a conventional Doherty amplifier 100 is constructed of a divider 101 which divides an input signal into two, a carrier amplifier 102 to which one of the divided signals from the divider 101 is inputted, a ¼-wave transmission line 104 to which the other divided signal is inputted, a peak amplifier 105 to which the output signal of the ¼-wave transmission line 104 is inputted, a ¼-wave transmission line 103 to which the output signal of the carrier amplifier 102 is inputted and a combiner 106 which combines the output signal of the ¼-wave transmission line 103 and the output signal of the peak amplifier 105.
For example, the carrier amplifier 102 operates class-AB or class-B biasing and the peak amplifier 105 operates class-C biasing. Furthermore, the peak amplifier 105 is set so as to operate only when the carrier amplifier 102 operates around the saturation output power. When the input signal current is sufficiently small, the Doherty amplifier 100 is operative to amplify the input signal using only the carrier amplifier 102. On the other hand, when the input signal current is at such a high level that the carrier amplifier 102 operates around saturation output power, the Doherty amplifier 100 operates to combine the output signals of the carrier amplifier 102 and that of the peak amplifier 105 by the combiner 106.
Because the peak amplifier 105 does not operate when the input signal current is small, the Doherty amplifier 100 thereby can reduce power consumption. Moreover, when the input signal current is larger than a predetermined current where the peak amplifier 105 just operates, the carrier amplifier 102 and the peak amplifier 105 operate simultaneously. The efficiency of the Doherty amplifier 100 is higher than that of the conventional amplifier which corresponds to the carrier amplifier 102. Therefore, the Doherty amplifier 100 can achieve highly efficient amplification as a whole.
The Doherty amplifier 100 has the ¼-wave transmission line 103 on the output side of the carrier amplifier 102. With this function of the ¼-wave transmission line 103, a load impedance at the output port of the carrier amplifier 102 changes depending on ON/OFF of the peak amplifier 105. This further improves the efficiency of the Doherty amplifier 100. Hereinafter, this reason will be explained in brief.
First, for simplicity of explanation, suppose that the ¼-wave transmission line 103 of the Doherty amplifier 100 is a lossless, distributed constant transmission line. Generally, the following relationship holds in a lossless transmission line.
                              [                                                                      V                  L                                                                                                      I                  L                                                              ]                =                              [                                                                                cos                    ⁢                                                                                  ⁢                    β                    ⁢                                                                                  ⁢                    L                                                                                                              j                      ·                                              R                        0                                                              ⁢                    sin                    ⁢                                                                                  ⁢                    β                    ⁢                                                                                  ⁢                    L                                                                                                                                          (                                              j                        /                                                  R                          0                                                                    )                                        ⁢                    sin                    ⁢                                                                                  ⁢                    β                    ⁢                                                                                  ⁢                    L                                                                                        cos                    ⁢                                                                                  ⁢                    β                    ⁢                                                                                  ⁢                    L                                                                        ]                    ⁡                      [                                                                                V                    0                                                                                                                    I                    0                                                                        ]                                              (        1        )            
V0 and I0 denote a voltage value and a current value at an input port of the lossless transmission line, respectively. Furthermore, VL and IL denote a voltage value and a current value at an output port of the lossless transmission line, respectively. β and L denote a frequency-dependent phase constant and a transmission line length, respectively. j and R0 denote an imaginary number unit and a characteristic impedance of the lossless transmission line, respectively.
In the case of the ¼-wave transmission line 103, since βL=π/2 is satisfied, the following relationship holds in the ¼-wave transmission line 103.
                              [                                                                      V                  L                                                                                                      I                  L                                                              ]                =                              [                                                            0                                                                      j                    ·                                          R                      0                                                                                                                                        j                    /                                          R                      0                                                                                        0                                                      ]                    ⁡                      [                                                                                V                    0                                                                                                                    I                    0                                                                        ]                                              (        2        )            
When an impedance seen looking into the output side of the Doherty amplifier 100 from the joint between the output port of the ¼-wave transmission line 103 and the peak amplifier 105 is R0/2, the ¼-wave transmission line 103 is set so that the characteristic impedance becomes R0.
When the input signal current of the Doherty amplifier 100 is smaller than the predetermined current and the peak amplifier 105 is OFF, an output impedance of the peak amplifier 105 ideally becomes infinite. In this case, the load impedance VL/IL at the output port of the ¼-wave transmission line 103 becomes R0/2. Therefore, from Equation (2),R0/2=VL/IL={j·R0·I0}/{(j·V0)/R0}=R02·(I0/V0)  (3)holds. Equation (3) can be modified as:V0/I0==2R0  (4)
This indicates that a load impedance at the input port of the ¼-wave transmission line 103, that is, that at the output port of the carrier amplifier 102 becomes 2R0.
On the other hand, when the input signal current is larger than the predetermined current and the peak amplifier 105 is ON, the carrier amplifier 102 and the peak amplifier 105 operate in parallel and the output signals of both amplifiers are combined. In this case, the load impedance VL/IL at the output port of the ¼-wave transmission line 103 becomes R0, while the load impedance at the output port of the peak amplifier 105 also becomes R0. Therefore, from Equation (2),R0=VL/IL={j·R0·I0}/{(j·V0)/R0}=R02·(I0/V0)  (5)holds. Equation (5) can be modified as:V0/I0=R0  (6)
This indicates that the load impedance at the input port of the ¼-wave transmission line 103, that is, that at the output port of the carrier amplifier 102 becomes R0.
When the peak amplifier 105 is OFF, the load impedance at the output port of the carrier amplifier 102 becomes 2R0 and when the peak amplifier 105 is ON, that at the output port of the carrier amplifier 102 becomes R0.
The carrier amplifier 102 is designed to offer smaller saturation output power and higher efficiency, when the load impedance at the output port of the carrier amplifier 102 becomes 2R0. As a result, when the input signal current is smaller than the predetermined current and the peak amplifier 105 is OFF, the Doherty amplifier can offer high efficiency amplification.
On the other hand, the carrier amplifier 102 and the peak amplifier 105 are designed such that when their load impedances at their output ports are R0, the Doherty amplifier 100 is designed to maximize the saturation output power. As a result, the Doherty amplifier 100 when the peak amplifier 105 is ON can offer larger saturation output power and more linear amplification than that when the peak amplifier 105 is OFF. Here, while the carrier amplifier 102 operates a saturation output power, the amount of current inputted to the peak amplifier 105 can be reduced accordingly; therefore, it is possible to further prevent the peak amplifier 105 from being saturated.
Non-Patent literature 1: W. H. Doherty, “A new high efficiency power amplifier for modulated waves”, Proc. IRE, Vol. 24, No. 9, pp. 1163-1182, September 1936.
In this way, the features of the conventional Doherty amplifier 100 are an ON/OFF operation of the peak amplifier 105 and a high efficiency amplification operation using an impedance conversion circuit by means of the ¼-wave transmission line 103. Here, as shown in Equation (1), the relationship between the impedance V0/I0 at the input port of the ¼-wave transmission line 103 and the impedance VL/IL at its output port depends on the frequency of the transmission signal (because the phase constant β depends on the frequency). In the conventional Doherty amplifier 100, the ¼-wave transmission line 103 is designed to perform a desired impedance conversion at one frequency (for example, the center frequency of the frequency band in which a signal should be amplified). Therefore, the ¼-wave transmission line 103 does not perform the desired impedance conversion at other frequency bands. In this case, the combination of the output of the carrier amplifier 102 and the output of the peak amplifier 105 is no longer optimal and the operation of the Doherty amplifier 100 becomes incomplete.
FIG. 11 shows the input impedance expressed by amplitude (Mag) and phase (Phase) on the carrier amplifier 102 side (Port1) of the ¼-wave transmission line 103 which is designed to perform a desired impedance conversion at design frequency 2 GHz. As shown in FIG. 11, the input impedance of the ¼-wave transmission line 103 designed in this way becomes design value 100 Ohm at design frequency 2 GHz, but it does not become 100 Ohm at other frequencies.
In this way, in the conventional Doherty amplifier 100, the frequency band in which the Doherty amplifier 100 operates is determined according to the design frequency of the ¼-wave transmission line 103. Therefore, the conventional Doherty amplifier 100 is unable to operate with sufficient gain and efficiency in a frequency band at a center frequency other than the design frequency (except the case where βL in Equation (1) is an even multiple of π/2).